The present invention relates to press sections in paper machines for dewatering paper webs and, more particularly, to closed press sections comprising a compact press roll combination in which several press nips are formed by various press rolls between which nips the web runs supported on the surface of a roll without substantial open draws.
The invention also relates to a frame construction for a paper machine press section, the press section comprising a compact press roll combination in which several press nips are formed by various rolls between which the web runs supported by a roll surface without substantial open draws.
Paper machine press sections generally comprise several press nips formed by pairs of rolls through which the paper web and dewatering fabrics, such as press felts or pairs of press felts, run. In this connection, reference is made to the press sections disclosed in Finnish Applications Nos. 781,426 (available to public, November 6, 1979) and 821,995 (available to public, December 5, 1983), and to U.S. Pat. No. 4,209,361, all assigned to the assignee of the instant application. Reference is also made to Canadian Pat. No. 1,068,525 and U.S. Pat. No. 4,075,056 of Beloit Corporation.
U.S. Pat. No. 4,209,361 discloses the well-known "Sym-Press II" press section currently finding widespread use by various paper manufacturers. Several advantages are obtained by using this press section which are of practical importance. One of the most important advantages is that the paper web can be passed through the entire press section as a closed draw supported on a solid surface, first between the first and second nips on a pick-up felt, over a sector of the press-suction roll and then on the smooth surface of the center roll of the press, usually a rock roll, and then through the third nip.
The general principles and theories of the pressing of paper webs will not be described since they are well-known to those skilled in the art. However, a brief discussion as to the particular problems that occur in the pressing of thin paper qualities is useful as a background to the present invention. A significant problem in the dewatering of thin paper qualities is the rewetting of the web. Such rewetting occurs in the "Sym-Press II" press section, particularly between the first and second nips in which the same felt, which also functions as a pick-up felt, is used as a press felt. On the other hand, the center rock roll does not rewet the web. Another problem in the pressing of thin paper qualities is that the pressure exerted on the web in any of the nips is limited. In the case of thin paper qualities, two-sided dewatering is not necessary when the dry solid content of the web is increased in the press section, such as from about 16% to about 45%, since the quantity of water to be removed corresponds to a water layer of only about 0.18 mm. The two felts in the first nip, together with the soft coating on the suction roll, causes a relatively low peak pressure in the first nips, which reduces their efficiency.
Presses of the "Sym-Press II" type have the characteristic that only one side of the web is pressed against the smooth surface of the rock roll or against a corresponding surface of synthetic material, so that the web becomes at least to some extent asymmetric. Although this does not have a major harmful effect on most paper qualities, it is not desirable in the case of thin paper qualities.